Microsoft made available two different Windows 10 test builds today, December 10. There's a Dev Channel build 21277 from the pre-release branch which includes the promised x64 app emulation for Arm, among other features. And there's a Windows 10 test build, No. 20277, that's available to the Dev Channel testers from the Iron (Fe) branch available today, as well.
The RS_Prerelease build 21277 -- which ultimately is expected to be designated as the "Cobalt" branch -- includes the features Microsoft had previously been testing but removed at the end of October. This includes the updated emoji picker, redesigned touch keyboard, voice typing, theme-aware splash screens and more. It also provides the aforementioned Arm emulation support.
Update: As Microsoft execs point out, the Dev Channel releases are not tied to a specific feature update these days. That technically means Microsoft could take some, all, or none of the features in the Prerelease build and move them into any Windows 10 feature update -- 21H1, 21H2 or later.
Currently, Windows on Arm (WoA) natively supports Arm apps, including ARM64 versions. But so far, only 32-bit Intel (x86) apps are supported in emulation. This lack of x64 emulation has limited the number of apps that can run on WoA devices, since apps that are 64-bit only have only been available on Windows on Arm (WoA) devices if and when developers created native versions of them. But now, x64 Arm apps also can run in emulation.
Before Insiders rush off to install the RS_Prerelease branch build 20277, they should read the fine print. These builds are currently offered as optional updates to "seekers," meaning Insiders will have to go and choose to install them. Once they do this, they can't go back to Iron; they'll continue to receive RS-Prerelease builds, going forward.
Those who don't want to do this can continue to automatically get builds from the Iron branch until Microsoft moves all Insiders to the Dev Channel to the RS-Prerelease builds "some time after the holidays."